1. Field of the Invention
This invention relates to a color printing apparatus that uses a print head for forming dots of a plurality of colors.
2. Description of the Related Art
Serial scan-type printers and drum scan-type printers are dot recording devices which record dots with a print head while carrying out scans both in a main scanning direction and a sub-scanning direction. There is a technique called xe2x80x9cinterlace schemexe2x80x9d, which is taught by U.S. Pat. No. 4,198,642 and Japanese Patent Laid-Open Gazette No. 53-2040, for improving the image quality of printers of this type, especially ink jet printers.
FIG. 27 is a diagram for explaining an example of the interlace scheme. In this specification, the following parameters are used to define a printing scheme.
N: Number of nozzles;
k: Nozzle pitch [dots];
s: Number of scan repeats;
D: Nozzle density [nozzles/inch];
L: Sub-scanning amount [dots] or [inch];
w: Dot pitch [inch].
The number of nozzles N is the number of nozzles actually used to form dots. In the example of FIG. 18, N=3. The nozzle pitch k is the interval between the centers of the recording head nozzles expressed in units of the recorded image pitch (dot pitch w). In the example of FIG. 27, k=2. The number of scan repeats is the number of main scans in which all dot positions on a main scanning line are serviced. In the example of FIG. 27, s=1, i.e., all dot positions on a main scanning line are serviced in a single main scan. When s is 2 or greater, the dots are formed intermittently in the main scanning direction. This will be explained in detail later. The nozzle density D (nozzle/inch) is the number of nozzles per inch in the nozzle array of the print head. The sub-scanning amount L (inch) is the distance moved in 1 sub-scan. The dot pitch w (inch) is the pitch of the dots in the recorded image. In general, it holds that w=1/(Dxc2x7k), k=1/(Dxc2x7w).
The circles containing two-digit numerals in FIG. 27 indicate dot recording positions. As indicated in the legend, the numeral on the left in each circle indicates the nozzle number and the numeral on the right indicates the recording order (the number of the main scan in which it was recorded).
The interlace scheme shown in FIG. 27 is characterized by the configuration of the nozzle array of the recording head and the sub-scanning method. Specifically, in the interlace scheme, the nozzle pitch k indicating the interval between the centers of adjacent nozzles is defined as an integer at least 2, while the number of nozzles N and the nozzle pitch k are selected as integers which are relatively prime. Two integers are xe2x80x9crelatively primexe2x80x9d when they do not have a common divisor other than 1. Further, sub-scanning pitch L is set at a constant value given by N/(Dxc2x7k).
The interlace scheme makes irregularities in nozzle pitch and ink jetting feature to thin out over the recorded image. Because of this, it improves image quality by mitigating the effect of any irregularity that may be present in the nozzle pitch, the jetting feature and the like.
The xe2x80x9coverlap schemexe2x80x9d, also known as the xe2x80x9cmulti-scan schemexe2x80x9d, taught for example by Japanese Patent Laid-Open Gazette No. 3-207665 and Japanese Patent Publication Gazette No. 4-19030 is another technique used to improve image quality in color ink jet printers.
FIG. 28 is a diagram for explaining an example of the overlap scheme. In the overlap scheme, 8 nozzles are divided into 2 nozzle sets. The first nozzle set is made up of 4 nozzles having even nozzle numbers (left numeral in each circle) and the second nozzle set is made up of 4 nozzles having odd nozzle numbers. In each main scan, the nozzle sets are each intermittently driven to form dots in the main scanning direction once every (s) dots. Since s=2 in the example of FIG. 28, a dot is formed at every second dot position. The timing of the driving of the nozzle sets is controlled so that the each nozzle set forms dots at different positions from the other in the main scanning direction. In other words, as shown in FIG. 28, the recording positions of the nozzles of the first nozzle set (nozzles number 8, 6, 4, 2) and those of the nozzles of the second nozzle set (nozzles number 7, 5, 3, 1) are offset from each other by 1 dot in the main scanning direction. This kind of scanning is conducted multiple times with the nozzle driving times being offset between the nozzle sets during each main scan to form all dots on the main scanning lines.
In the overlap scheme, the nozzle pick k is set at an integer at least 2, as in the interlace scheme. However, the number of nozzles N and the nozzle pitch k are not relatively prime, but the nozzle pitch k and the value N/s, which is obtained by dividing the number of nozzles N by the number of scan repeats, are set at relatively prime integers instead.
In the overlap scheme, the dots of each main scanning line are not all recorded by the same nozzle but by multiple nozzles. Even when the nozzle characteristics (pitch, jetting feature etc.) are not completely uniform, therefore, enhanced image quality can be obtained because the characteristics of the individual nozzles is prevented from affecting the entire main scanning line.
However, what is the preferred printing scheme in terms of improving the quality of the printed image differs depending on the arrangement of the print head nozzle array. This means that for a specific print head, it can be difficult to set a printing scheme for improving the quality.
Accordingly, an object of the present invention is to provide a printing technique that makes it possible to obtain high image quality with a specific print head.
The present invention is directed to a printing technique using a printing apparatus having a sub-scanning drive section includes a first sub-scanning drive mechanism that effects sub-scan feeding at a relatively high precision, and a second sub-scanning drive mechanism that effects sub-scan feeding at a relatively low precision after completion of sub-scan feeding by at least the first sub-scanning drive mechanism. A print head is provided with a first array of a plurality of dot formation element groups that are arrayed in a prescribed order in the sub-scanning direction. The first array includes a group of yellow dot formation elements for forming yellow dots. The plurality of dot formation element groups of the first array are arrayed in an order that is determined so that at an arbitrary point on the print medium yellow dots are formed after dots of other colors. Each of the plurality of dot formation element groups has a mutually equal number of dot formation elements When the print medium is being fed in a sub-scanning direction not by the first sub-scanning drive mechanism but by the second sub-scanning drive mechanism, printing in the vicinity of the trailing edge of the print medium is effected using the group of yellow dot formation elements but not the other groups of the first array.
In accordance with this invention, printing in the vicinity of the trailing edge of the print medium is effected using only the yellow dot formation elements of the first array that are used to form yellow dots. In the vicinity of the trailing edge sub-scan feeding of the print medium is effected not by the first sub-scanning drive mechanism but by the second sub-scanning drive mechanism, which has a relatively low feed precision. However, yellow dots are relatively inconspicuous, so even though the sub-scanning feed precision is lower, it does not have much of an adverse effect on image quality. Thus, the invention makes it possible to execute printing that enables high image quality to be obtained in respect of the specific print head.
When the print medium is being fed proximate the trailing edge of the print medium in a sub-scanning direction not by the first sub-scanning drive mechanism but by the second sub-scanning drive mechanism, sub-scanning feeding may be effected by the second sub-scanning drive mechanism at the same feed amounts by which feeding has been effected by the first sub-scanning drive mechanism. This enables the printing process to be continued without adjusting the sub-scan feeding, thereby simpifying control of the scanning.
The print head may further include a second array of dot formation elements, disposed parallel to the first array, having a group of black dot formation elements for forming black dots. The second array may be arranged to form dots on the print medium prior to the first array. The group of black dot formation elements includes a plurality of dot formation elements disposed at the same sub-scanning positions as the plurality of dot formation element groups of the first array. During color printing the formation of black dots is implemented using only black dot formation elements located at the same sub-scanning positions as chromatic color dot formation elements in use of a specific chromatic color dot formation element group of the first dot formation element array, where the specific chromatic color dot formation element group is a group that can print dots before the other dot formation element groups of the first array. Thus, at each location on the print medium black dots are formed earlier than dots of other colors, which prevents bleeding of the black dots and thereby makes it possible to obtain color images of a high chroma.
The first and second arrays may formed within an identical actuator. As it thus becomes possible to position adjacent dot formation elements with good precision, image quality can be improved.
The present invention is also directed to a print head for use in a printing apparatus that prints images by forming dots on a print medium. The print head comprises: a first array of a plurality of dot formation element groups that are arrayed in a prescribed order in a sub-scanning direction. The first array includes a group of yellow dot formation elements for forming yellow dots. Each of the plurality of dot formation element groups has a mutually equal number of dot formation element. The group of yellow dot formation elements is arranged at an end of the first array. With this print head, yellow dots can be formed in the vicinity of a trailing edge of a print medium after dots of other colors are formed thereon. Since yellow dots are relatively inconspicuous, even if sub-scanning feed precision is lower in the vicinity of the trailing edge of the print medium, it does not have much of an adverse effect on image quality.
Specific aspects of the invention can be applied to various types of printing apparatus, printing methods, computer. program products, and print heads.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.